linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for AmpereOne

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Message-ID: <99cee26b-351e-4bc3-81a8-ec8ced373770@arm.com>
Date: Wed, 2 Oct 2024 10:59:04 +0100
From: Robin Murphy <robin.murphy@....com>
To: Yang Shi <yang@...amperecomputing.com>, Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...pe.ca, will@...nel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation
 for AmpereOne

On 2024-10-01 8:48 pm, Yang Shi wrote:
> 
> 
> On 10/1/24 12:29 PM, Nicolin Chen wrote:
>> On Tue, Oct 01, 2024 at 12:09:03PM -0700, Yang Shi wrote:
>>> On 10/1/24 11:27 AM, Nicolin Chen wrote:
>>>> On Tue, Oct 01, 2024 at 11:03:46AM -0700, Yang Shi wrote:
>>>>> Using 64 bit immediate when doing shift can solve the problem.  The
>>>>> disssembly after the fix looks like:
>>>> [...]
>>>>
>>>>>           unsigned int last_sid_idx =
>>>>> -               arm_smmu_strtab_l1_idx((1 << smmu->sid_bits) - 1);
>>>>> +               arm_smmu_strtab_l1_idx((1UL << smmu->sid_bits) - 1);
>>>> Could a 32-bit build be a corner case where UL is no longer a
>>>> "64 bit" stated in the commit message?
>>> It shouldn't. Because smmu v3 depends on ARM64.
>>>
>>> config ARM_SMMU_V3
>>>          tristate "ARM Ltd. System MMU Version 3 (SMMUv3) Support"
>>>          depends on ARM64
>> ARM64 can have aarch32 support. I am not sure if ARM64 running a
>> 32-bit OS can be a case though, (and not confined to AmpereOne).
> 
> I don't think ARM64 runs 32-bit kernel, at least for newer kernel.

Just use ULL - if the point is that it must be a 64-bit shift for 
correctness, then being clear about that intent is far more valuable 
than saving one character of source code.

Thanks,
Robin.

> 
>>
>>>> Then, can ssid_bits/s1cdmax be a concern similarly?
>>> IIUC, ssid_bits is determined by IDR1_SSIDSIZE. It is GENMASK(10, 6). So
>>> it shouldn't be 32. IDR1_SIDSIZE is GENMASK(5, 0).
>> Rechecked the RM. Yea, max sid can be 32 but max ssid is 20 at
>> this moment, so we should be safe.
>>
>> Thanks
>> Nicolin
>